In recent years, a solar cell has been widely employed as a device directly converting solar energy into electric energy from the viewpoints of effective use of natural resources and prevention of environmental pollution. Further developments of solar cells are in progress with the object of power generation efficiency and durability.
As shown in FIG. 1, in general, the solar cell is produced by laminating a light receiving side transparent protection material 11 such as a glass substrate, a light receiving side sealing film 13A, a photovoltaic elements 14 such as crystalline silicon cells, a backside sealing film 13B and a backside protection material (back cover) 12 in this order, degassing in evacuated state, crosslinking and curing the light receiving side sealing film 13A and backside sealing film 13B, and adhesively combining them.
In order to generate a large electrical output, the solar cell has plural photovoltaic elements 14 connected to each other by using a connecting tab(s) 15. Therefore, photovoltaic elements are sealed by using insulating sealing films 13A, 13B so as to maintain insulation property for photovoltaic elements 14.
In the related art, as a sealing film for such solar cells, a film made of ethylene-vinyl acetate copolymer such as ethylene-vinyl acetate copolymer (occasionally abbreviated to EVA) or ethylene-ethyl acrylate copolymer (EEA) is used. Especially, the EVA film is preferably used because it has low cost and high transparency. Furthermore, in the EVA film used for the sealing film, a crosslinker such as an organic peroxide other than EVA is used to improve crosslink density.
For instance, Patent Literature 1 discloses a solar cell sealing film in which an organic peroxide is used as a crosslinker and EVA of the sealing film is crosslinked up to 80 to 90% gel rate by heat in a step for adhesively combining with each member. As the result, the heat resistance of the solar cell is improved.